Device for the non-dispersive optical determination of the concentration of gas and smoke components

ABSTRACT

A method and apparatus for non-dispersive optical determination of gas and smoke components in a mixture by reflection and detection of radiation, of a different wavelength for each component, through the mixture, wherein the radiation is periodically intercepted before it enters the mixture and reflected direct to the photo-receiver and the measurement signals are divided by the stored direct signals.

The invention relates to a method for the non-dispersive opticaldetermination of the concentration of gas and smoke components in amixture of various gases and possibly smoke, in which radiation of atleast as many different wavelengths as there are gas components andpossibly a smoke component in the mixture to be measured, is conductedthrough the mixture, reflected, conducted through the mixture again andpicked up at successive times periodically by a photo-receiver, fromwhose output signals, representing the particular transmissionconcerned, the concentrations of the components are computed by Beer'sLaw.

The basis for each concentration measurement on the principle ofradiation absorption is Beer's Law, according to which the transmissionof an irradiated sample depends exponentially on the product of themeasured length L and the sample concentration c in accordance with thefollowing formula:

    T = I (λ)/Io (λ) = exp. [- k (λ) × L × c](1)

Here Io (λ) and I (λ) are radiation intensities of a given wavelength atthe beginning and end, respectively, of the absorption length. k (λ) isthe wavelength-dependent absorption coefficient of the examined gascomponent and it often happens that a number of gas components absorbdifferently at the same point of the spectrum, this being manifest asthe so-called cross-sensitivity in the measurement.

Since measurement of radiation intensities is generally carried out withone radiation detector the measurement is falsified not only byvariations in the radiation intensity of the source of radiation andradiation losses in the optical path but also by changes in thesensitivity of the radiation detector. The continuous variations in themeasured radiation intensities, independently of absorption losses inthe measurement length, are also designated as the drift of thearrangement.

The known non-dispersive gas analysers can be divided, irrespective ofthe spectral range employed, into two groups, that is to say equipmentutilising optical filter arrangements or selective radiation sources forwavelength selection and equipment effecting spectral separation by wayof gas-filled vessels.

To obviate the so-called cross-sensitivity (by this is to be understoodthe influencing of absorption at a certain point on the spectrum by adifferent gas which likewise absorbs at this point) with equipment ofthe second type, the work is carried out with a dual beam arrangement,wherein radiation emanating from one or two sources passes throughspatially separated measuring and comparing vessels which are chargedwith the gas sample to be examined, and a neutral gas, respectively. Thedifferent radiation absorption in the two vessels is measured inradiation detectors specially constructed according to the spectralrange for producing electric signals. There are also single beamarrangements in which a single vessel is filled alternatively with testgas and with reference gas, but this is very complicated.

A detailed account of this type of gas analyser can be found in thearticle "Neuester Stand der Entwicklung von Kontrollmessgeraten zurDaueruberwachung von Gas-Emissionen" ("Latest state of development ofmonitoring and measuring apparatus for the continuous monitoring of gasemissions") by H. W. Thoenes and W. Gruse, which appeared in theperiodical "Staub-Reinhaltung der Luft" (Maintenance of dust-free air),28 (1968), Pt. 3, pp. 128-134.

In their use for continuous monitoring of emission these equipments havethe main disadvantage of requiring a system for the removal and thepreparation of the test gas. Apart from the complexity of this method,errors may occur in gas removal so that the partial flow obtained formeasurement is no longer representative of the waste gas. Disturbingforeign substances, such as solid particles, water vapour and aerosolsmust be removed from the flow of measurement gas so that the withdrawaland cleaning system with its connecting lines does not become clogged orcorroded. The filters, coolers and receivers used in preparation of thetest gas must not affect or adulterate the components being measured.

In a further known arrangement (DT-OS 2,324,049) which seeks to obviatethese difficulties by direct measurement in the duct carrying the wastegas, for example a stack, the radiation source, optical filter unit,detector and electronic evaluating system are brought together in ahousing outside the stack, while projecting into the stack from the sideis a gas-permeable probe at the end of which the radiation is deflectedby a mirror and thus arrives back in the measuring head. Measuring iscarried out in the ultraviolet portion of the spectrum by thecombination of two wavelengths respectively for the gas components SO₂and NO₂ as well as electronic evaluation of the two transmission valuesto determine the gas concentration. For balancing and for the regularobtaining of a neutral and calibration point the probe is cleansed witha neutral gas which does not contain the components being measured.

In another known arrangement (DT-OS 2,340,747) the radiation source andalso the detector and evaluation unit are mounted opposite one anotherin separate housings on the stack and interconnected by way of agas-permeable pipe. The radiation source consists of two hollow cathodesof different emission wavelengths, tuned to the ultraviolet SO₂absorption band, the intensity of which is measured and evaluated in thedetector unit after the passage through the stack. A secondphotodetector is provided in the transmission part to pick up anychanges in the radiating capacity, which may occur in the two hollowcathode lamps.

There is also a report on dispersive and non-dispersive radiationanalysers measuring directly in the waste gas stack in an articleentitled "Monitoring Boiler Stack Gases" by T. C. Elliott in theperiodical "Power" (April 1975), pp. 92-94.

A separate arrangement of transmitter and receiver unit on the stack ishighly sensitive to adjustments, just like deflection of rays within aprobe by means of a mirror, and so the intensity of radiation passingover the measured distance depends substantially on the geometricalorientation and stability of the arrangement on the stack. Of course itremains possible to measure the gas content by the two-wavelength methodso long as both radiation components follow the same optical path inwhat is always the same mixture in space. On the other hand it isscarcely possible to measure the proportion of smoke in the waste gaswith arrangements of this type because solid particles other than a gascomponent reduce the transmissivity for both wavelengths and thereforerequire the unattenuated radiation as a reference. Every variation inradiation intensity which is due to adjustment would appear as ameasuring error in the determination of the solid content. Moreovercross-sensitivity to other gas components cannot be excluded even withthe two-wavelength method; it exists over wide ranges of the infra redspectrum through the absorption bands of water vapour and must also betaken into consideration in the ultraviolet range owing to the widenitrogen dioxide bands.

The equipments measuring directly in the stack according to the presentstate of the art allow for no special arrangements or measures tocompensate for the aforementioned cross-sensitivity. For continuousmonitoring of emissions long-term stability is also an importantrequirement of the equipment which can only be satisfied with greatdifficulty when operating with a plurality of beam transmitters orreceivers. Again, the formation of quotients in measuring with twowavelengths compensates drift effects in the optical and electro-opticalcomponents only in so far as they are not dependent on wavelength.Finally, the known method with direct measurement in the waste gas stackhas the disadvantage that the probe which is introduced covers only apart of the cross-section of the stack which need not necessarily berepresentative of the total flow of waste gas.

The aim of the invention is therefore to create a method and a device ofthe type referred to above which will obviate the aforementioneddisadvantages of the known equipment and permit a substantially moreaccurate measurement of concentration by means of radiation absorption,while eliminating in particular the disadvantages of the spectrallydifferent drift effects.

As a solution of this problem the invention provides that, atpredetermined intervals of time and for at least one period ofreception, the beams shall, even before entering the mixture, bereflected over the same beam path direct to the photo-receiver and theelectric reference signals obtained for the various test wavelengthsshall be stored during the successive predetermined interval of time andthat the output signals representing the respective transmission shall,before computation of the concentrations, be divided by the storedreference signal obtained with the same wavelength. The idea underlyingthe invention is, thus, the compensation of the wavelength-dependentinfluences of the spectral sensitivity of the photo-receiver and of thespectral emissivity of the source of radiation as well as any otherinfluences of the optical components. Thus the drift effects arecompletely eliminated by the method according to the invention and,indeed, even when they are dependent on wavelength. Only the tworeflectors must be stable and controlled with exclusion of gasabsorption.

In general it is sufficient if the reference signals are formed anew andstored every 10 minutes because in shorter intervals the drift effectsare practically unnoticeable. In principle even longer intervals wouldbe adequate for the formation of the reference signals. With theelectronic holding circuits at present employed in analogue techniquesthe 10-minute cycle is preferred; with digital circuitry it is possibleto choose longer intervals.

It is particularly advantageous if both the measuring and the referencesignals can be regulated to the same level for the same basicconditions. This can be achieved, for example, by differentamplification of the individual signals and it leads to particularlysimple computational evaluation.

Where the mixture consists of smoke, SO₂ and NO₂ the measurementwavelengths are conveniently 313, 435 and 546 nm, since the associatedabsorption coefficients of the said components yield transmissionsignals which can be evaluated well for the concentrations generallypresent in waste gas stacks and where there is complete radiation of thestack.

The invention also has as its subject a particularly convenient devicefor carrying out the aforementioned method with a lighttransmitter-receiver for the beams disposed at one end of themeasurement distance and a reflector at the other end of the measurementdistance, also a computing circuit to determine the concentrations fromthe electric signals formed by the light receiver, characterized inthat, before entering the measured distance, the beam of light comingfrom the light transmitter-receiver can be brought optionally on to areference reflector which reflects the beam in the same direction as thereflector disposed at the end of the measured distance. As a result ofthis arrangement the reference signals needed for drift compensation caneasily be formed between the test periods by projecting the beam oflight on to the reference reflector.

It is convenient if the reference reflector can be swung into the pathof the beam between the light transmitter-receiver and the measureddistance. In a practically advantageous form of construction thereference reflector is fixed to a lever arm articulated immediatelybeside the front objective of the light transmitter-receiver, and anactuating rod is articulated preferably at a relatively short distancefrom the pivot of the lever arm and can be driven by a motor through acrank in one direction or the other. In this way, with relatively smallmovements, big excursions of the lever arm carrying the referencereflector can be obtained, thus permitting a rapid swing-in which is,however, controlled and free from violent impact.

The reflectors are preferably retroreflectors, especially triplemirrors, and the one disposed at the end of the measured distance, atleast, should be radiated all round by the light beam.

This form of construction is particularly convenient when the lighttransmitter-receiver and the retro-reflector are disposed on oppositesides of the stack, when as is known, varied expansions of the stackwalls due to heating may result in considerable distortion. However, asa result of the construction according to the invention with radiationover the retroreflector these distortions have no adverse effect on themeasurement.

A particularly simple optical arrangement together with goodinsensitiveness to adjustment is obtained by using an autocollimationbeam path.

For measuring a smoke component, an SO₂ component and an NO₂ component alow-pressure mercury-vapour lamp is especially suitable because it hasemission bands at the measuring wavelengths 313, 435 and 546 nm.

Selection of wavelength is effected conveniently by a filter wheel whichhas on a first circumference filters which arrive in succession in thereception beam path. In measurements of smoke, SO₂ and NO₂ and also whena low-pressure mercury-vapour lamp is being used it is then sufficientto construct the filters of combinations of tinted glass which, as isknown, are relatively wide banded. However, in view of the emissioncharacteristic of the low-pressure mercury lamp wide-band filters ofthis type are sufficient.

It is also convenient to provide a dark zone also in the filter sequenceto define the neutral point in the electronic evaluation.

There are further provided preferably on a second circumference of thefilter wheel control slots co-operating with a light barrier and amaster clock, each of which is associated with a filter or the darkzone. Here, the master clock effects the blanking of the particularwavelength signal which is just formed at a certain moment.

Finally, on a third circumference there is provided a control apertureco-operating with a light barrier and the master clock, by means ofwhich the re-setting of the master clock and hence an indication of thestart of the respective cycle is obtained.

The photo-receiver is advantageously connected by way of a pre-amplifierto a channel balancing stage which has two balancing amplifiers for eachmeasuring wavelength. With suitable balancing by the amplifiers it ispossible, in the absence of smoke and test gas, to boost all the testand reference signal levels to the same value, which is advantageous tolater evaluation.

In a further suitable arrangement the channel balancing stage is soconstructed, utilising a certain operating range of the computingcircuit, to be described hereinafter, that, economising on balancingamplifiers, only a paired coincidence of measuring and reference signallevels is adjusted in the absence of smoke and test gas for one testwavelength at a time, whereby optimally, with additional optical finebalancing for one wavelength with the aid of the iris diaphragm at theabove-described reflectors, only two balancing amplifiers are needed forthe channel balancing stage.

It is preferable if a balancing amplifier is also provided for the darkzone in the channel balancing stage.

In a further advantageous form of construction an electronic switchcontrolled by the master clock is connected to the channel balancingstage and stored the wavelength, test and reference signals and, whereapplicable, also the dark zone signal individually in holding circuits.

Then the computing circuit is connected to the switch and interrogatesin pairs the measurement and reference signals associated with the samewavelength and divides one by the other. This division represents thecritical step according to the invention because all parametersimpairing the measurement through drift effects hereby drop out.

To arrive at the concentration data, logarithms are taken appropriatelyin the computing circuit of the signals which have been divided one bythe other.

The logarithm signals representing the extinctions are preferably storedin holding circuits.

Finally there is a computing stage connected to the holding circuitswhich from the logarithm signals forms the concentration signals whichcan be displayed on any kind of indicating instrument.

The invention is described below by way of example with reference to thedrawing in which the Figures are as follows:

FIG. 1 is a diagram of a preferred device and circuit for carrying outthe method of the invention;

FIG. 2 is a schematic front elevation of the filter wheel used in thedevice of FIG. 1;

FIG. 3 is the front elevation of a practical embodiment of the lighttransmitter-receiver of the device shown in FIG. 1;

FIG. 4 is a pulse diagram of the photo-receiver controlled by the filterwheel;

FIG. 5 is a schematic representation of a modified arrangement of thereference reflector which may be used instead of the laterally swingablereference reflector shown in FIGS. 1 and 3;

FIG. 6 is a section of the triple reflector used in the device accordingto the invention;

FIG. 7 is a schematic block circuit diagram of the computing circuitused in the device according to the invention; and

FIG. 8 is a schematic side elevation of a modified part of the deviceaccording to the invention shown in FIG. 1 which operates withalternating light.

As shown in FIG. 1 the device according to the invention has a lighttransmitter-receiver 11 fixed to one side of a stack 72 and a reflectorhead 12 fixed to the opposite side of the stack 72 and consisting of ahousing 14 and a retro-reflector 13 disposed on its end face. The stack72 has apertures 74, 75 for the passage of the light beam in the regionof the light transmitter-receiver 11 and the reflector head 12.

The housing 14 of the reflector head 11 and a connecting branch 76 whichconnects the housing 15 of the light transmitter-receiver 11 to thestack 72 are provided with supply pipes for scavenging air 71 throughwhich scavenging air is blown in, in the direction of the arrow f, toprevent impurities getting from the stack 72 into the connecting branch76 or the front objective 20 and becoming deposited there.

Fitted in the housing 15 of the light transmitter-receiver is aradiation source 16, preferably constituted by a low-pressuremercury-vapour lamp, which by way of a condensing lens 19 and abeam-splitting mirror 77 fully illuminates a front objective 20 disposedin the end wall of the housing 15. From the front objective 20 thereemerges an almost parallel and preferably slightly divergent beam oflight 50 which traverses the connecting branch 76, the stack 72permeated by flue gases 49 and finally the housing 14 of the reflectorhead 13 to impinge on the retroreflector 13 which preferably consists oftriple mirrors. It is important that at the site of the reflector 13 thelight beam 50 is of a greater diameter than the retroreflector so as togive all-round irradiation of the reflector 13 as can be seen fromFIG. 1. In this way the flow of light thrown back from the reflector 13undergoes no change in the event of certain relative shifts or tiltingsof the optical axis between the reflector head 12 and the lighttransmitter-receiver.

Owing to the ratio of dimensions of the light beam 50 and theretroreflector 13, the reflected beam 78 has a smaller diameter than thebeam 50 -- likewise almost parallel -- which leaves the transmitter.

The reflected beam 78 is concentrated, by the objective 20 and afterreflection at the beam-splitting mirror 77, on a photo-receiver 17 infront of which is disposed, according to the invention, a filter wheel18 with a rotational axis 79 disposed parallel to the incident light.The filter wheel 18 is driven with rotary motion by a motor 80.

A control filter 73 can also be pushed in in front of the filter wheel,as shown in FIG. 1, which corresponds to a predetermined distribution ofthe components of the waste gases 49 and serves to check the functioningcapacity of the device.

As shown in FIG. 2, the filter wheel 18 comprises as essential elementsthree filters 23, 24, 25 which are elongated in the circumferentialdirection and extend in each case over an angle of a little under 90°and which, when a low-pressure mercury-vapour lamp 16 is used, for thesource of radiation, consist simply of combinations of tinted glass, thefunction of which is to pass only one of the three wavelengths 313 nm(filter 23), 435 nm (filter 24) and 546 nm (filter 25), stopping theother two wavelengths in each case.

For the filter 23 a combination of tinted glass UG 11/1 mm and GG 10/1mm is used conveniently, for the filter 24 a combination of tinted glassBG 3/1 mm and NG 3/1 mm and for the filter 25 a combination of tintedglass OG 515/1 mm and NG 3/1 mm. The designations indicate filters madeby the firm of Schott.

Thus according to the invention, by using a selective radiation source16 for spectral separation it becomes unnecessary to employ expensiveand in UV only poorly transparent interference filter, as would benecessary with a wide-band source of radiation. In fact, the highlytransparent and cheap combinations of tinted glass are adequate.

As can be seen from FIG. 2, the fourth quadrant of the filter wheel 18is provided with a dark zone which defines the dark current of thephoto-receiver 17 and thereby constitutes a base for measurement of thequantity of light passed by the filters 23, 24, 25.

As shown in FIG. 1, for one revolution of the filter wheel 18, thefilters 23, 24, 25 and the dark zone 26 come in succession into the beampath to the photo-receiver 17.

On another circumference of the filter wheel 18 lying further inward aredisposed slots 34 which are elongated in the circumferential directionand likewise extend in each case over somewhat less than 90°, each ofwhich is associated with one of the filters or the dark zone 26respectively. The arrangement in space does not have to be as shown inFIG. 2; it depends upon the arrangement of the light barrier 35co-operating with the slots 34 along the circumference. The function ofthe circumferential slots 34 is to generate a clock signal whichactivates in a suitable way an electronic system, to be described later,while the associated filter, or the dark zone as the case may be, iseffective.

Finally, on a third circumference situated between the first and secondones there is provided further a small, round control aperture 36 whichco-operates with a further light barrier 37, shown schematically in FIG.1, and serves to generate a re-setting signal for the master clock andhence to mark the start of the respective cycle.

Instead of the slots 34 and the control aperture 36 it is also possibleto provide appropriate reflective marks which co-operate with reflectivelight barriers. Furthermore, to increase the number of gas components tobe measured, the number of filters disposed in the filter wheel 18 mayalso be increased. In the form represented and with the previouslydefined filters 23, 24, 25, the filter wheel permits the determinationof the smoke components and the gas components SO₂ and NO₂ in the wastegas 49 in the stack 72.

Thus, on rotation in the direction of the arrow F in FIG. 2, thereflected light beam 78 is split in succession into three wavelengths546 nm, 435 nm and 313 nm. As will be described in detail below it isimportant, for simplicity of evaluation, that the gas component SO₂absorbs only at wavelength 313 nm. To exclude cross-sensitivity to thevery wide-band absorption of NO₂ for the SO₂ measurement and in order tobe able to indicate the concentration of this component itself, it ismeasured directly with light of the wavelength 435 nm. The influence ofthe solid content in the flue gas on measurement at these wavelengthscan also be eliminated by formation of a reference signal at wavelength546 nm as will be described more fully below.

The photo-receiver 17 is preferably a photo-multiplier.

For complete drift compensation there is provided between the lighttransmitter-receiver 11 and the intake opening 74 in the stack 72according to the invention a reference reflector 22 which isconveniently constructed as a triple reflector in exactly the same wayas the reflector 13. As shown in FIG. 1 the reference reflector 22 isnormally disposed beside the beam path. However, it can be moved in thedirection of the double arrow f' into the position represented bydash-dot lines in FIG. 1 within the beam path so that the light beam 50incident upon it is reflected back upon itself. The most convenientarrangement is that shown in FIG. 1, that is to say the referencereflector 22 is situated immediately in front of the front objective 20of the light transmitter-receiver 11 when it is pushed into the beampath. The scavenging air introduced at 71 thus also keeps the referencereflector 22 free from any contamination.

So long as there is no risk, especially of wavelength-dependent driftthrough the front objective 20 itself, which could be the case in anumber of applications, the reference reflector 22 may also be disposedin the interior of the housing 15, that is to say behind the frontobjective 20.

FIG. 3 shows a front elevation of the light transmitter-receiver 11 witha preferred swinging arrangement for the reference reflector 22. Thereference reflector 22 is fastened to a lever arm 58 which isarticulated immediately beside the front objective 20 with a rotationalaxis 59 running parallel to the optical axis. By means of an operatingrod 60 which is indicated only schematically, which acts on the leverarm 58 at a short distance from the rotational axis 59, it is possible,by way of a crank 61 driven by a motor -- not shown -- to exert a torqueon the lever arm 58 which causes the reflector 22 to swing into the beampath out of the front objective 20.

However, as shown in FIG. 5, a reference reflector 22' running at theside of the beam path and parallel to it may, for example, be fixed anda mirror 40 deflects the light beam 50 towards the reference reflector22'.

Now, in order to be able to act optionally on the measuring reflector 13provided at the end of the measured distance 49 and the referencereflector 22', it is possible either to make the deflecting mirror 40capable of being pushed or swung in the direction of the double arrow f"out of the parallel beam 50 or to construct the mirror 40 as apermanently fixed beam-splitting mirror which thus allows through a partof the incident light beam 50 and deflects another part towards thereference reflector 22'. To ensure that at a certain time only one ofthe two reflectors 13 or 22' is being acted upon a swivelling diaphragm39 may be provided which either interrupts the light flow to theretro-reflector 13 in the position shown in solid lines in FIG. 5 or,after swinging round on a swivel path 48, comes into the position 39'shown by broken lines where the light beam can reach the measuringreflector 13 but the light to the reference reflector 22' is interruptedby the swivelling diaphragm 39'. In this form of construction it isparticularly advantageous to use as the fixed beam-splitting mirror 40the beam splitter 77 which is already mounted in thetransmitter-receiver housing. The condensing lens 19 would then take theplace of the objective 20. An advantage here is the smaller moved massof the diaphragm 39'. Because of the use of the triple reflector 22' noproblems are presented by the adjustment.

Disposed at any point on the part which can move at the change over frommeasurement to comparison, for example on the reference reflector 22 inFIG. 1, is a contact piece or a cam 29 which co-operates with acontactor 30 in such a way that when the reference reflector 22 ispushed in, a signal relative to this is passed to a master clock 28. Inother words the master clock 28 can be aware at all times whether thereference reflector 22, 22' is effective or not.

The following section describes the electronic construction of thedevice according to the invention.

Disposed on the photo-receiver 17 is a ballast resistor 38, which ismade to measure, from which is tapped a voltage which is supplied to apre-amplifier 21.

The output of the pre-amplifier has a reinforced voltage U_(V) which issupplied to a channel balancing stage which is of great importance tothe invention. The output signal of the pre-amplifier 21 is conducted inparallel to seven amplifiers 41, 42, 43, 44, 45, 46 and 47 with balance.This division of the output signal of the voltage amplifier 21 alreadypresupposes a subsequent channel separation.

The amplifiers 41, 42, 43 are associated with the three wavelengths 313,435 and 546 nm with the reference reflector 22, 22' swung into the beampath. The amplifiers 44, 45, 46 are associated respectively with one ofthe three above-named wavelengths with the reference reflector 22 swungout of the beam path, that is to say with an effective measuringreflector 13.

The seventh amplifier 47 with balance belongs to the dark zone 26 of thefilter wheel and serves to create a base for measurement at theindividual wavelengths with the measuring or the reference reflector inthe beam path.

The output signals of the amplifiers 41 to 47 with balance are passedindividually to an electronic switch 27 which is controlled by a masterclock 28 which picks up control signals U_(R) from the light barrier 37and U_(T) from the light barrier 35 at the filter wheel 18 and alsoestablishes by way of the contactor 30 the effectiveness ornon-effectiveness of the reference reflector 22.

A holding circuit 51, 52, 53, 54, 55, 56 is associated with eachmeasuring wavelength both for effective and also for non-effectivereference reflector 22 in the switch 27. A seventh holding circuitstores the signal representative of the dark current and conducts it byway of an impedance transducer 32 back to the input of the channelbalancing stage 31 in order to form a base.

Scanning by means of the master clock 28 is carried out as follows,reference being made to the pulse diagram in FIG. 4.

At the end of a scanning cycle the control aperture 36 reads out a pulsesignal, shown schematically in the bottom diagram in FIG. 4, via thelight barrier 37, which signal indicates as signal U_(R) to the masterclock 28 that a new scanning cycle must start.

Now if the dark zone 26 enters the path of the beam to thephoto-receiver 17, the holding circuit 57 in the switch 27 is connectedto the balancing amplifier 47 so that by way of the impedance transducer32 a defined base is created for subsequent measuring operation asindicated in the top diagram in FIG. 4 by D.

As soon as the filter 25 enters the beam path the control slot 34associated therewith, through the release of a second pulse signal U_(T)to the light barrier 35 (middle diagram in FIG. 4), causes the balancingamplifier 46 to be connected to the holding circuit 56. At the end ofthe first pulse U_(T) the balancing amplifier 47 had already beendisconnected from the holding circuit 57.

Now, by way of the balancing amplifier 46, the measuring signal producedby reflection at the measuring reflector 13 is stored in the holdingcircuit 56 at wavelength 546 nm as shown in the top diagram in FIG. 4.As the filters 24 or 23 enter the beam path this cycle is repeated and,as can be seen in the top diagram in FIG. 4, the signals U_(V) for thewavelengths 435 nm or 313 nm are stored in the holding circuits 55 or54, respectively.

These cycles are repeated continuously during measuring, the outputsignals 7, 3, 2 and 1 of the master clock 28 connecting the holdingcircuits 57, 56, 55 and 54 cyclically with the balancing amplifiers 47,46, 45, 44.

At intervals of about 10 minutes the reference reflector 22 is pushedinto the beam path. The contacts 29, 30 hereby signal to the masterclock 28 that the change over from "measuring" to "comparison" has beenmade. The manner of operation of the master clock 28 consequentlychanges so that its outputs 7, 6, 5 and 4 connect the holding circuits57, 53, 52 and 51 cyclically to the balancing amplifiers 47, 43, 42 and41, respectively. The pulse image is then substantially the same as inFIG. 4 with the sole difference that the heights of the reference pulsesassociated with the individual wavelengths are different, and indeedhigher, by comparison with the "measuring" setting.

Reference figures in brackets show, in the pulse diagram in FIG. 4, towhich control apertures or filter zones of the filter wheel in FIG. 2the individual pulses are assigned.

Thus at the output of the switch 27 there are constantly six signalsavailable at the holding circuits 51 to 56, of which the signals U₁ ',U₂ ', and U₃ ' correspond to the received signals for the individualwavelengths 313, 435 and 546 nm with the reference reflector 22 pushedin, whereas the signals U₁, U₂ and U₃ are representative of thewavelength signals 313, 435 and 546 nm with the measuring reflector 13operative.

By reason of the balancing possibilities in the amplifiers 41 to 47 itis possible, by carrying out a balancing operation before putting theequipment into use, to bring all the output signals of the holdingcircuits 51 to 56 to the same level. This applies not only to theindividual wavelength signals by comparison with one another but inparticular to the output signals which appear when the referencereflector is brought into action, on the one hand, and when themeasuring reflector is operative, on the other. In this way, forexample, it is also possible to compensate in the simplest and mostdirect manner differing reflection characteristics of the measuringreflector 13 and the reference reflector 22.

To obtain a coarse balancing between measurement and reference signalsit is also possible according to the invention to provide in front ofthe reflectors rigidly adjustable iris diaphragms (81 in FIG. 6) bymeans of which both reflectors can be pre-adjusted substantially to thesame intensity of reflection.

Evaluation of the signals U₁ 'to U₃ takes place in a computing circuit33 which will now be described with reference to FIG. 7.

As shown in FIG. 7 the signals U₁ ', U₂ ', U₃ ', U₁, U₂ and U₃ arepassed to an interrogation circuit 62 which contains a switch 82 with aninternal master clock by means of which, in the manner illustratedschematically in FIG. 7, the measurement and reference signals U₁ ' U₁or U₂ ', U₂ or U₃ ', U₃ are passed in succession in pairs to a followingdivision and logarithmic stage 63. There then appears at the output ofthis stage 63 a signal E₁, E₂ or E₃ depending on which of the threepairs happens to be at the stage 65. These signals E_(i) are passed to adistribution stage 64 which is also controlled by the internal masterclock built into the switch 82 by way of a lead 83, representedschematically. By means of this control the extinction signals E₁, E₂and E₃ representative of the individual wavelengths are passed toholding circuits 65, 66 or 67 in which they are stored. For the holdingcircuits it is preferable to use capacitor stores with operationalamplifiers.

In another suitable form of construction it would be possible to omitthe switch 82 and in three separate division and logarithmic stages 63to carry out simultaneously a paired processing in each case of themeasurement and reference signals U₁ ', U₁, or U₂ ', U₂ or U₃ ', U₃,which would permit a direct further processing of the extinction signalsE₁, E₂ and E₃ in the computing stage 68 to be described later withoutany previous intermediate storage in the holding circuits 65, 66 and 67.

As a result of the formation and processing of the signals picked up bythe photo-receiver according to the invention the transmission signalsstored in the holding circuits 65, 66 and 67 are totally independent ofdrift and free from cross-sensitivity. This can be attributed to thefact that all components such as spectral sensitivity of thephoto-receiver 17 and spectral emission capacity of the radiation source16 are contained in the reference signals U₁ ', U₂ ', U₃ ', so thatthese components which cause the drift drop out when division isperformed. Because of this division also, variations in the transmissionbehaviour of the optical components used show no disturbingmanifestations in the measuring process. In particular, ageing phenomenain the lamp and the multiplier no longer affect the measurement. Itshould be emphasised in particular that comparison is always made withone and the same wavelength, thus excluding disturbances due todifferent drift at different wavelengths.

The channel balancing in the stage 31 corresponds to an electronicsmoothing of the equipment characteristic which is determinedsubstantially by the radiation source 16 and the photo-receiver 17. Aremarkable fact is that, besides differences in the reflection behaviourof the measurement and the reference reflector 13, 22, it is alsopossible to compensate differences of a spectral character individuallyfor the individual wavelengths, which is a considerable simplificationof reflector production and adjustment.

Connected on to the holding circuits 65, 66, 67 is a computing stage 68in which the concentrations of smoke, SO₂ and NO₂ are calculated fromthe corrected extinctions values E₁, E₂ and E₃ which are independent ofdrift.

In measurement of smoke, SO₂ and NO₂ in the stack 72 and with the use ofthe wavelengths 313, 435 and 546 nm, the following relationships areobtained for the logarithmic transmission values designated asextinctions E_(i) :

    E.sub.1 = E.sub.R + .sup.1 k.sub.SO.sbsb.2 = c.sub.SO.sbsb.2 × L + .sup.1 K.sub.NO.sbsb.2 × c.sub.NO.sbsb.2 x l        (2)

    E.sub.2 = E.sub.R + .sup.2 k.sub.NO.sbsb.2 × c.sub.NO.sbsb.2 × l                                                         (3)

    E.sub.3 = E.sub.R + .sup.3 k.sub.NO.sbsb.2 × c.sub.NO.sbsb.2 × L                                                         (4)

in these formulae E_(R) stands for the smoke extinction, c theconcentration of the gas characterised by an index, k the spectralabsorption coefficient for the gas concerned at the above-mentionedwavelengths 1, 2 or 3 and L stands for the length of the measureddistance. The smoke extinction E_(R) is related to the dust content asfollows:

    E.sub.R = k.sub.r × C.sub.r × L.

allowing for the fact that L and k are constants a gauge for theconcentration of NO₂ is obtained from the following calculation:

    ƒ(c.sub.NO.sbsb.2) = E.sub.2 - E.sub.3 = E.sub.2 = L × c.sub.NO.sbsb.2 (.sup.2 k.sub.NO.sbsb.2 - .sup.3 k.sub.NO.sbsb.2) (5)

a gauge for smoke concentration is obtained from the followingcalculation:

    ƒ(c.sub.n) = E.sub.3 - β × E.sub.2 = E.sub.3 = E.sub.R (60)

where

    β= .sup.3 k/(.sup.2 k- .sup.3 k)                      (7)

Finally, the concentration of SO₂ can be determined from the followingformula:

ƒ(c_(SO).sbsb.2) = E₁ - γ × ₂ - E₂ - E₃ = L × c_(SO) ₂ × k_(SO).sbsb.2(8)

where

    γ = (.sup.1 k - .sup.3 k) /(.sup.2 k- .sup.3 k)      (9)

In order to carry out the above-mentioned calculations the output of theholding circuit 67 is connected via an inverting stage 84 and a resistor85 to one input of a differentiating stage 86. The output of the holdingcircuit 66 leads via a resistor 87 to the inverting input of anoperational amplifier 88 which at the same time is connected via theregulating resistor 89 to the output of the inverting stage 84.

The output of operational amplifier 88 is also connected via aregulating resistor 90 to the other input of the differentiation stage86.

The output of the holding circuit 65 is applied via a resistor 94 to theinput of the operational amplifier 91 which is also connected viaregulating resistors 92 and 93 to the outputs of the inverting stage 84and the operational amplifier 88. By means of this circuit there appearsat the output of the operational amplifier 91 a signal representative ofthe SO₂ concentration, at the output of the operational amplifier 88 asignal representative of the NO₂ concentration and at the output of thedifferentiation stage 86 a signal representative of smoke.

The above values of the coefficients β and γ are set permanently at theabove-defined figures by the ratio of the resistors 92 and 94 or 85 and90. The balancing which is effected once only is carried out on asection free from test gas and smoke in a simple manner in that with thecontrol filter 73 swung in the concentration values represented therebyare indicated.

Then, as shown in FIG. 1, there can be connected to the three outputs ofthe computing circuit 33 an indicating instrument 69 which, for example,after voltage-current conversion can take the form of a triple recorderwith a respective indicating range 0 to 20 mA in accordance withpredetermined concentration ranges so that variations in concentrationcan be monitored continuously.

As can be seen from FIG. 1, the source of radiation 16, the drives forthe filter wheel and reference reflector system, the lamps in the lightbarriers and also the whole electronic system are supplied from a commonpower-supply unit.

The arrangement of the reference reflector outside the actual housingfor the optical system 15 as shown in FIG. 3 has the advantage that thetransmission characteristics of the exit window, which may vary throughformation of a deposit, for example, are also included in the driftcompensation.

Information on the position of the reference reflector for the masterclock can be given by means of a simultaneously rotating cam plate whichswitches on a relay as soon as the reference reflector 22 is in the beampath.

In FIG. 6 the triple reflector used according to the invention 13 or 22is of a special construction. Usually, retro-reflectors are constructedon the triple principle in the form of ground or pressed triple mirrorsof glass or plastics which the light enters via a plane surface andafter total reflection at the internal triple surfaces emerges againdeflected in itself with a certain lateral transposition. Triple mirrorsof this type are of little use in the invention since the necessaryUV-transmission can be obtained hardly at all with pressed plasticstriple reflectors and only at great cost with ground quartz glass triplereflectors.

As can be seen from FIG. 6 a less expensive but still very efficientarrangement can be obtained by providing the back of a conventionaltriple plastics pressing 95 with an aluminum vapour-deposited coating 96after a preliminary cleaning operation. The surface obtained in this wayacts in the same way as a conventional triple reflector but it alsoreflects in the same way in the ultraviolet range. By means of anadditional vapour-deposition with a special UV-permeable protectivelayer of magnesium fluoride any changes in reflection characteristicsdue to ageing of the aluminum coating are effectively prevented. Thetriple reflector 95 thus produced is held in a mount 97 in the mannershown so that the sensitive triple tips are left free. An iris diaphragm81 disposed in front of the triple reflector 95 can be drawn up to agreater or lesser extent to vary the reflecting surface and fixed in thefinal position.

It is convenient if the whole arrangement is covered by a window 98which transmits the ultraviolet.

Under certain conditions it may be favourable to construct the gasanalyser with an additional zero scanning of the radiation intensity, asexplained below with reference to FIG. 8, showing an alternating lightarrangement wherein, in addition to the usual chopper 99 driven by amotor 100, the chopper apertures 101 of which can be broughtsubstantially into the focal point of the condenser 19, a filter wheel18' is disposed on the same shaft 102. Here the construction of theoptical arrangement is such that there is an intermediate imaging at thesite of the chopper and thus the diaphragm apertures can be kept small,as a result of which a number of radiation pulses can be obtained foreach filter pass. A similar effect can be obtained by pulsing theradiation source and this is possible without any trouble with thelow-pressure mercury-vapour lamps which are used. In any case thesearrangements need a matched electronics construction with alternatingcurrent connection and band-pass filters in their input section.

FIG. 8 is intended to demonstrate that the invention is not restrictedto the arrangement of the filter wheel 18 which is represented inFIG. 1. Moreover the above description makes it clear that the methodand the device according to the invention can be used to determine morethan three gas or smoke components in a mixture of gases, for which atany given time as many radiations of different wavelength required asthere are components to be determined.

We claim:
 1. Apparatus for non-dispersive optical determination of theconcentrations of gas and smoke components in a mixture of gasescontaining smoke, comprising a light transmitter-receiver for emittingand receiving reflected radiation at one end of a measured distancethrough the mixture, said light transmitter-receiver being provided witha front objective, a reflector at the other end of the measureddistance, a reference reflector arranged to periodically reflect emittedradiation, a photo-receiver in said light transmitter-receiver forreceiving radiation reflected by said reference reflector before saidradiation enters the mixture, a plurality of wave-length filters, saidradiation being passed periodically at successive intervals through atleast as many different of said wave-length filters as there arecomponents to be measured during a first predetermined time period, saidphoto-receiver receiving radiation passed through the mixture, reflectedby said reflector, passed through the mixture again and passedperiodically at successive intervals through said filters during asecond time period, a computer for determining the concentrations of thecomponents in said mixture from electrical signals generated by saidphoto-receiver and fed to said computer, the computer storing differentelectrical reference signals for the various test wave-lengths obtainedduring said first time period and during said second time period anddividing each of the electrical signals influenced by the transmissionat the associated wavelength individually by the stored reference signalobtained at the same wavelength, so as to obtain a continuouslycorrected extinction signal for each wavelength, and transforming thesaid extinction signals into concentration signals through Beer's Law.2. Apparatus according to claim 1, wherein the reference reflector canbe swung into the beam path between the light transmitter-receiver andthe measured distance.
 3. Apparatus according to claim 1 wherein thereference reflector is fastened to a lever arm for moving the same to aposition immediately adjacent the front objective of the lighttransmitter-receiver.
 4. Apparatus according to claim 3, wherein at ashort distance from the fulcrum of the lever arm there is situated anoperating rod which can be driven in both directions by a motor througha crank.
 5. Apparatus according to claim 1, wherein each reflector is aretroreflector.
 6. Apparatus according to claim 5, wherein theretroreflector is a vapour-deposited aluminium coating on a plasticstriple pressing.
 7. Apparatus according to claim 5, wherein at least thereflector disposed at the end of the measured distance is irradiated onall sides by a light beam emitted by said light transmitter-receiver. 8.Apparatus according to claim 1, wherein an autocollimation beam path isemployed.
 9. Apparatus according to claim 1, wherein a low-pressuremercury-vapour lamp is employed as a light source in said lighttransmitter-receiver.
 10. Apparatus according to claim 1, whereinwavelength selection is effected by a filter wheel which has, on a firstcircumference, filters which successively receive light in the form ofreflected radiation.
 11. Apparatus according to claim 1, wherein alow-pressure mercury-vapour lamp is employed as a light source in saidlight transmitter-receiver and the filters consist of combinations oftinted glass.
 12. Apparatus according to claim 10, wherein in thesequence of said filters there is also provided a dark zone on saidfirst circumference of said filter wheel.
 13. Apparatus according toclaim 12, wherein there is further provided a master clock, and on asecond circumference of said filter wheel there are provided controlslots co-operating with a light barrier and said master clock, each ofwhich slots being associated with one of said filters and said darkzone.
 14. Apparatus according to claim 13 wherein on a thirdcircumference of said filter wheel there is provided a control aperturecooperating with a second light barrier to generate a re-setting signalfor said master clock.
 15. Apparatus according to claim 1, wherein thephoto-receiver is connected through a pre-amplifier to a channelbalancing stage which has a balancing amplifier for measuring eachwavelength.
 16. Apparatus according to claim 15, wherein a balancingamplifier is also provided in the channel balancing stage for dark zonesignals.
 17. Apparatus according to claim 15, wherein an electronicswitch controlled by a master clock is connected to the channelbalancing stage which switch stores wavelength measuring and referencesignals received by said photo-receiver, and also where applicable darkzone signals, individually in holding circuits.
 18. Apparatus accordingto claim 17, wherein a computing circuit is connected to said electronicswitch which circuit interrogates in pairs and divides by one anotherthe measuring and reference signals belonging to the same wavelength.19. Apparatus according to claim 18, wherein logarithms are taken in thecomputing circuit of the signals which are divided by one another. 20.Apparatus according to claim 19, wherein the logarithm signalsrepresenting the extinctions are stored in holding circuits. 21.Apparatus according to claim 20, wherein a computing stage is connectedto the holding circuits, which stage forms the concentration signalsfrom the logarithm signals.
 22. Apparatus according to claim 1, whereinpre-set iris diaphragms are disposed on at least one of said measuringreflector and reference reflector.
 23. Apparatus according to claim 1,wherein operations are carried out with alternating light.
 24. Apparatusaccording to claim 23, wherein a chopper and a filter wheel are disposedon the same shaft.
 25. Apparatus according to claim 15, wherein thechannel balancing stage has two balancing amplifiers for each measuringwavelength.
 26. Apparatus according to claim 1, wherein wavelengthselection is effected by a filter wheel which has, on a firstcircumference, filters which successively receive light in the form ofreflected radiation, said sequence including a dark zone; on a secondcircumference, control slots so-operating with a light barrier and amaster clock, each of which slots is associated with one of said filtersand dark zone; and on a third circumference, a control apertureco-operating with a second light barrier and the master clock. 27.Apparatus according to claim 1, wherein the photo-receiver is connectedthrough a pre-amplifier to a channel balancing stage which has abalancing amplifier for each measuring wavelength and a balancingamplifier in the channel balancing stage for dark zone signals, anelectronic switch controlled by a master clock connected to the channelbalancing stage which switch stores wavelength measuring and referencesignals received by said photo-receiver and also where applicable thedark zone signals, individually in holding circuits; a computing circuitbeing connected to said electronic switch which circuit interrogates inpairs and divides by one another the measuring and reference signalsbelonging to the same wavelength.
 28. Apparatus according to claim 27,wherein logarithms are taken in the computing circuit of the signalswhich are divided by one another and the logarithm signals representingthe extinctions are stored in holding circuits, a computing stage beingconnected to the holding circuits, which stage forms the concentrationsignals from the logarithm signals.
 29. Apparatus according to claim 26,wherein each reflector is an aluminium coated retro-reflector and atleast said reflector disposed at the end of the measured distance isirradiated on all sides by a light beam, emitted by said lighttransmitter-receiver, the path of said beam being autocollimated.